The invention relates to a tunnel kiln especially for firing ceramic workpieces. As used herein, the term tunnel kiln is understood to mean all types of continuously operating kilns with a kiln channel or kiln chamber.
In the known tunnel kilns the material to be fired is transported through the furnace chamber by means of kiln cars, in the direction of the longitudinal axis of the kiln. The kiln cars thus pass through a heating , a firing and finally a cooling zone. The kiln cars are provided with a bottom designed like a plate, made of thermal insulating, heat-resistant material, thereby forming themselves kiln sole sections movable through the firing channel. This known construction of a kiln car is pretty strong and offers a safe protection against excessive heating of the leading means for the kiln cars. As a result, the bottom plate of each kiln car, forming respectively one kiln sole section of the firing channel, has to be moved through all zones of the kiln together with the workpieces to be fired and placed there upon. Regarding known kilns for ceramic workpieces, the relation between the net feeding weight per kiln car to the corresponding tare weight is at about 1:1.8, i.e. the weight for the workpieces to be treated is merely about 36% of that of the total mass to be heated up and cooled thereafter. As a consequence of this, high expenditure of energy is necessary and because of the necessity to heat and to cool a considerable tare mass together with the workpieces to be treated a substantial inertia during the heating and cooling process is caused with considerable limitations of the passage of carriage through said tunnel kilns. In case the known tunnel kilns of said type are provided with recuperators in the cooling zone, the heat transmission from the bottom plates of the kiln cars to the recuperator is more difficult and unfavourable in its effectiveness.